1. Field of the Invention
The present invention relates to a process for producing polyolefins having special rheological and compatibilization properties, as well as to the resulting polyolefins and to their use.
The problems that arise with polymers in general, and polyolefins in particular, relate to their insufficient melt strength when they are being processed by extrusion.
It is well known that the melt strength of polyethylene (PE) and of polypropylene (PP), which is defined by a high elongational viscosity, is insufficient in certain types of processing, such as extrusion foaming, extrusion blow-moulding, thermoforming and blow moulding, particularly 3D blow moulding.
Solutions proposed for solving this problem consist in making the macromolecular structure of the PE or PP branched by creating covalent bonds between the macromolecules. However, in practice, branched resins manufactured by covalent coupling all suffer from a tendency of the branches to degrade due to the effect of shear inherent in the processing. Moreover, significant irreversible covalent branching results in melt fractures, limiting the productivity and/or quality of the finished product.
In order to be able to increase the connection density between macromolecules without being limited by crosslinking, it is possible to provide a substantial part of the branches via reversible ionic bonds. This makes it possible to increase the melt strength while retaining the thermoplastic character
2. Description of the Related Art
The process disclosed in patent EP 0,086,159 proposes the crosslinking of xcex1-olefin polymers and copolymers in order to improve their melt strength characteristics. The grafting of a carboxylic acid in the presence of a radical generator and the subsequent salt formation by metal compounds are envisaged.
The objetive of the present invention is to provide a process for the production of polyolefins having improved properties, especially with regard to melt strength.
The present invention consequently relates to a process for producing polyolefins, comprising the following steps:
a) grafting of acid groups onto polyolefins by. means of a graftable monomer bearing at least one functional group chosen from a carbonyl and an acid anhydride, optionally in the presence of another graftable monomer bearing a vinyl unsaturated group and, optionally, one or more aromatic rings;
b) purification, consisting in removing at least some of the grantable monomer bearing at least one functional group chosen from a carbonyl and an acid anhydride that has not reacted with the polyolefins;
c) neutralization of the acid groups by at least one neutralizing agent.
The polymers obtained by the process according to the present invention have improved properties, especially with regard to melt strength, by virtue of the introduction of an additional purification step b). This is because in the case of processes of the prior art, a relatively large proportion of functional graftable monomers remains in the final product, these monomers not having reacted with the polyolefins. The presence of these functional graftable monomers which have not reacted with the polyolefins, that is to say which have not been grafted onto the polyolefins after step a), may be responsible inter alia for insufficient melt strength and for colour problems. The objective of this purification step b) is consequently to remove at least some of the functional graftable monomers that have not reacted with the polyolefins.